Isomerization alkyne/allene

The first investigations in the 1960s [11,12] established the base-induced isomerization of alkyne precursors as the most practical and general route for the synthesis of alkoxy-and aryloxyallenes. In the meantime, a number of monosubstituted allenes 8 bearing an achiral or a chiral group R is smoothly accessible by this efficient procedure (Scheme 8.5) [1, 2,13-19]. Beside the most commonly used base potassium tert-butoxide, other bases, e.g. n-butyllithium, are also applicable for this isomerization. Recently, the yields of alkyne-allene isomerizations could be significantly increased, in particular with aryloxy-substituted allenes, by using microwave irradiation (Eq. 8.1) [20]. 

Kanematsu and co-workers devised a simple pathway to isobenzofurans and dihydroisobenzofurans starting with substituted furans of type 62. Treatment with strong base results in an alkyne-allene isomerization. Subsequent intramolecular cycloaddition, ring opening (probably oxygen lone-pair assisted), and acidic workup give 63. 

Muller and coworkers have recently developed a coupling-isomerization reaction, initially identified as a side reaction which occurred under standard Sonogashira conditions [79]. As demonstrated below, the coupling reaction is followed by a shuffling of oxidation states via an alkyne-allene isomerization [80]. The product, a,P-unsaturated ketone 146, is reminiscent of a product which would be obtained from a Heck reaction. The utility of this reaction was further demonstrated when diamine 147 was added to the reaction pot. Following a conjugate addition reaction and imine formation, compound 148 resulted from the three-component, one-pot reaction sequence enabled by the coupling-isomerization reaction. 

Reactions at the Propargylic (a) Carbon 4.43.1 Alkyne/Allene Isomerization... 

Of further significance is the fact that no 1,3-pentadiene is formed This behavior is similar to that of the butynes, where also no 1,3-butadiene was observed. Furthermore, this is in complete accordance with the proposed mechanism of the potassium 3-aminopropylamide-mediated isomerization of internal alkynes to terminal alkynes by repetitive alkyne-allene-alkyne isomerizations [24]. 

Such a behavior cannot always be guaranteed the heterocycle 32 with a terminal alkyne and an allene isomerizes both fuctional groups, the benzylic alkyne to an allene and the benzylic allene to a 1,3-diene, both now in conjugation with the het-eroarene [46] (Scheme 1.12). 

As with the reaction of pyrroles, diazoles and triazoles react with propargyl bromide to yield TV-substituted products and, depending upon the base strength, either TV-prop-2-ynylazoles or allenic derivatives are formed [30]. Generally, with potassium carbonate under soliddiquid two-phase conditions at room temperature in the absence of a solvent, the prop-2-ynyl compounds are formed as sole products, whereas with solid potassium hydroxide at elevated temperatures the allenes are obtained as the major products. Benztriazole produces a mixture of the N1- and N2-prop-2-ynyl, and N2-allenic derivatives, whereas with potassium hydroxide only the N -allenic derivative is obtained. The alkynes readily isomerize to the allenes in the presence of base and the quaternary ammonium salt, or upon treatment with methanolic sodium hydroxide. A series of l-(alk-2-ynyl)imidazoles have been prepared, as intermediates in the synthesis of imidazopyridines [31 ] and the reaction of 3-hydroxymethylpyrazoles with propargyl bromide leads to pyrazolooxazines [32]. 

The double phosphinylation of propargylic alcohols with diphenylphos-phine oxide to form 2,3-bis(diphenylphosphinyl)-1-propenes is catalyzed by a thiolate-bridged diruthenium complex (Scheme 28) [69]. It has been shown that the reaction proceeds via three ruthenium-catalyzed transformations propargylation of the phosphine oxide, alkyne to allene isomerization, and addition of phosphine oxide to the allene structure. 

The conclusions from the above study have recently been largely confirmed by work of Saussey et They studied a series of alkynes and isomeric allenes... 

Alkynes and isomeric allenes also undergo partial equilibration after ionization. Generally, the smaller ions exhibit the largest rearrangement proclivities. 

Because of their lower stability relative to isomeric alkynes, allenes are generally only minor products of alkyne-forming dehydrohalogenation reactions. 

Isomerization of alkynes to allenes and then to conjugated dienes is catalyzed by Pd(0) species such as Pd(OAc)2 3.nd PhjP or Pdi(dba)3. The conjugated ketoalkyne 286 was converted into the conjugated dienone 287[141,I42]. 

Since cumulenes and alkynes are often easily interconvertible, many syntheses discussed above have allenic counterparts, especially base-catalyzed cyclizations of allenic alcohols.77 And, of course, several of the alkyne-based syntheses may well have allenic intermediates. There are, however, a few syntheses based specifically upon allene chemistry. In an important one, due to Stirling and his collaborators,78 an allenic sulfonium salt reacts with an enolate anion. Scheme 12 sketches the main features yields as high as 86% are recorded. Methoxyallene is easily metallated by butyllithium and so converted into an allenic epoxide that can be isomerized by fe/T-butoxide into a furan (Scheme 13) or an exocyclic equivalent similar to 15 clearly this method is particularly suited to the preparation of 3-methoxyfuran... 

Silyl esters have also been transformed successfully [137] another example reports the O-acylation of serine with a /l-alkynyl carboxylic acid anhydride accompanied by an in situ isomerization of the alkyne to the allene [138],... 

Allenes from Prototropic Isomerizations of Alkynes as Reactive Intermediates... 

Shioiri and co-workers developed a catalytic asymmetric synthesis of allenes by isomerization of the alkyne 240 to allene 242 under the control of a chiral phase-transfer catalyst 241 (Scheme 4.62) [98], Although the enantiomeric excess is not high (35% ee), this is the first example of the asymmetric isomerization of alkynes under phase-transfer catalyzed conditions. 

Scheme 4.62 Asymmetric allene synthesis via isomeration of alkyne 240 with the chiral phase-transfer catalyst 241.

In another reduction, the propargylic phosphate 64 is reduced with samarium(II) iodide in the presence of tetrakis(triphenylphosphine)palladium and tert-butanol as a proton source the allene 65 is produced almost exclusively, <1% of the isomeric alkyne 66 being present in the product mixture [19]. 

In the case of carboxylic acids [33], ketones [47] or sulfones [49], it was possible to prove a further prototropic rearrangement of allenes 19 with R2 = H yielding the alkynes 20. In other examples, the prototropic isomerization of the triple bond leads to conjugated dienes directly thus the allene of type 19 could only be postulated as an intermediate [50]. Braverman et al. showed that the allenes generated by prototropic isomerization of dipropargylic sulfones or sulfoxides, for example 24, are unstable. They are transferred rapidly via diradical intermediates to polycycles such as 27 [51-53],... 

The isomerization reactions of alkynes via [2,3]- or [3,3]-sigmatropic rearrangements are probably among the most important methods available for the introduction of the allene group into an organic molecule. The usefulness of this synthetic method [61, 62] and the access to new functionalized allenes [63] have been reviewed recently. 

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